1. Field of the Invention
This invention concerns the structure for a choke, in general, and more specifically deals with an improved choke structure that is especially applicable to high pressure fluids containing abrasive materials.
2. Description of the Prior Art
A choke for controlling the pressure of high pressure fluids is basically very simple and well known. However, in the case of fluids which include highly abrasive material particles therein, there have been various proposals for dealing with the abrasive wearing action. Such proposals, insofar as is known, have all accepted the fact of rapid wear and have consequently dealt with the manner of replacing the worn part or parts. Thus, for example, (while it is not a choke, per se) there is a patent concerning a nozzle used for sand blasting purposes that is described in U.S. Pat. No. 2,332,407. That patent deals with the structure for such a nozzle that has replaceable elements in the tapered section of the nozzle, so that the parts may be renewed after wear has taken place. Similarily, there is a U.S. Pat. No.2,407,050 which deals with the structure for a choke. It also is concerned with structure for making the element that is subject to wear replacable after the wearing action has taken place.
Furthermore, there is a known commerical choke structure that is supplied by FMC Corporation, Well Head Equipment Operation, P.O. Box 3091, 1777 Gears Road, Houston, Tex. 77001. That known choke structure includes a multiple stepped reduction in the inside diameter of its interior passageway leading to a small diameter choke passage. In addition, there is some internal threading to accompany an insert that may have a smaller passage. Such an insert is commonly called a choke bean. However, this has been found not to withstand highly abrasive flow under pressure, any better than other known types of commercial chokes. Even ceramic lined positive chokes which are available commerically, have been found not to withstand the abrasion by sharp sand and silt which is flowed by producting wells in an Athabasca tar sands thermal recovery project.
Consequently, it is an object of this invention to provide a choke, and a choke assembly which may be employed in the highly abrasive conditions such as found in flowing wells of an Athabasca tar sand thermal recovery procedure without being "washed-out" in relatively short time and so in need of replacement.